Method for producing polyacetal copolymer

ABSTRACT

A high-quality polyacetal copolymer produced by a simple process and in an economical manner. The method includes supplying a raw material including trioxane or the like to a reaction device; obtaining a reaction mixture by carrying out a polymerization reaction of the raw material in the state where the rotating shafts are inclined by 1 to 6° upwards with respect to the horizontal direction H from the introduction opening towards a polyacetal copolymer recovery portion; vaporizing and separating an unreacted monomer from the reaction mixture using a vaporization and separation portion; re-supplying the unreacted monomer to the reaction device; and recovering the polyacetal copolymer from the reaction using a polyacetal copolymer recovery portion.

This application is the U.S. National Phase under 35 U.S.C. §371 ofInternational Application PCT/JP2014/059957, filed Apr. 4, 2014,designating the U.S., and published in Japanese as WO 2014/175042 onOct. 30, 2014, which claims priority to Japanese Patent Application No.2013-092810, filed Apr. 25, 2013, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a method for producing a polyacetalcopolymer.

BACKGROUND ART

Until now, as a production method for a polyacetal copolymer, a cationiccopolymerization with trioxane as a main monomer, and a cyclic etherand/or cyclic formal having at least one carbon-carbon bond as acomonomer has been known. As the cationic active catalyst used for thesecopolymerizations, a Lewis acid, in particular halides of boron, tin,titanium, phosphorous, arsenic, and antimony, for example borontrifluoride, tin tetrachloride, titanium tetrachloride, phosphorouspentachloride, phosphorous pentafluoride, arsenic pentafluoride, andantimony pentafluoride, and their complex compounds or salts; protonicacids, for example perchloro acids; esters of protonic acids, inparticular esters of perchloro acids and lower aliphatic alcohols, forexample perchloro acid-tertiary butyl ester; anhydrides of protonicacids, especially mixed anhydrides of perchloro acids and loweraliphatic carboxylic acids, for example acetyl perchlorate, ortrimethyloxoniumhexafluorophosphate, triphenyl-methylhexafluoroacetate,acetyltetrafluoroborate, acetylhexafluorophosphate, andacetylhexafluoroarsenate, and the like have been proposed. Among these,boron trifluoride or coordination compounds of boron trifluoride andorganic compounds, for example ethers, are the most common aspolymerization catalysts with trioxane as the main monomer, and arewidely used industrially.

However, with commonly used polymerization catalysts such as borontrifluoride type compounds, in the later phase of polymerization thepolymerization speed suddenly decreases, and it is nearly impossible toobtain polymer conversion ratio of near 100% in a short time, and a verylong time is required, which is inefficient, and moreover, in the laterphase of polymerization, decomposition of the generated polymer by thecatalyst becomes relatively dominant, which not only causes a reductionin molecular weight, but also has the effect of degrading the qualitiessuch as heat resistance and the like. Further, if the amount of thepolymerization catalyst is increased, overall the polymerization speedis enhanced, and the polymer conversion ratio is also increased, but thequality of the generated rough polymer degrades more and more, and in alater step a complex stabilization treatment is required, whereby theproduction operation overall is by no means a preferable method.

Accordingly, the technique of adding a solution comprising a catalystdeactivation agent at a stage where the polymer conversion ratio isrelatively low to stop the polymerization, then washing, recovering andpurifying and reusing the remaining unreacted monomers, is widelypracticed.

Further, there have been various proposals for improving thepolymerization apparatus and the method of supplying the catalyst, inaddition to increasing the polymer conversion ratio. For example, atechnique of increasing the polymer conversion ratio for a device byinclining the polymerization device 1 to 10° (Patent Document 1), atechnique of providing a weir at the discharging port of thepolymerization apparatus (Patent Document 2), as well as a technique ofmixing the catalyst with the comonomer in advance, and supplying this tothe trioxane (Patent Documents 3 and 4) have been proposed. Thesetechniques are all effective for increasing the polymer conversion ratiofor the case of using a boron trifluoride type polymerization catalyst.

Further, a method has also been proposed to deactivate the rough polymerin the later stage of the polymerization by using a high activity,non-volatile polymerization catalyst, to directly recover the unreactedmonomer without carrying out washing, and reuse the same (PatentDocument 5). In this technique, it is possible to carry out recovery ofthe monomers directly from the rough polymer before deactivation, whichwas difficult with the boron trifluoride system commonly used in theprior art, and moreover, secondary reactions at a stage where a highconversion ratio has been achieved do not readily occur compared to aboron trifluoride type polymerization catalyst, whereby it is possibleto obtain a rough polymer which is excellent in thermal stability withlittle unreacted monomers by extremely simple steps.

Patent Document 1: Japanese Examined Patent Application Publication No.H05-008725

Patent Document 2: PCT International Publication No. WO1996/013534

Patent Document 3: Japanese Unexamined Patent Application, PublicationNo. H11-255854

Patent Document 4: Japanese Unexamined Patent Application, PublicationNo. H11-124422

Patent Document 5: Japanese Unexamined Patent Application, PublicationNo. H09-278852

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, in the case of adding a solution comprising a deactivationagent of the catalyst and stoping the polymerization at a stage wherethe polymer conversion ratio is relatively low, washing, recovering andpurifying, and reusing the remaining unreacted monomer, the unreactedmonomer is recovered as a solution where the concentration is relativelylow, and therefore, in order to reuse the unreacted monomer, it isrequired to use complex operations of separation and purification, aswell as energy. On the other hand, abandoning the recovery of theunreacted monomer becomes a total loss, and neither way is economicallypreferable.

Further, in the techniques disclosed in Patent Documents 1 to 4, theobtained rough polymer is one which comprises at least 10 wt % ofunreacted monomer, and in practical applications, it is required to gothrough deactivation and washing steps, moreover, for boron trifluoridetype polymerization catalysts, the inevitability of secondary reactionssuch as decomposition and the like at the stage where a high conversionratio has been achieved still remains a problem.

Further, in the technique disclosed in Patent Document 5, there is roomfor improvement in increasing the quality of the polyacetal copolymerand reducing costs.

Accordingly, the present invention has the objective of producing a highquality polyacetal copolymer economically and with a simple process.

Means for Solving the Problems

The present inventors, in order to solve the above problems, as a resultof repeated diligent study of polymerization reactions of trioxane,discovered that by carrying out the polymerization reaction of the rawmaterials under the conditions of (a) using a non-volatile protonic acidas the polymerization catalyst, and (b) inclining the rotating shafts ofthe reaction device 1 to 6° upwards with respect to the horizontaldirection from the input opening of the reaction device towards theoutput opening, when compared to the case of using a boron trifluoridetype polymerization catalyst of the prior art, it is possible to obtaina remarkably high polymer conversion ratio, as well as a high qualitypolyacetal copolymer, and thus completed the present invention.Specifically, the present invention provides the following.

(1) The present invention is a method for producing a polyacetalcopolymer comprising a raw material supply step of supplying a rawmaterial comprising trioxane, a comonomer which copolymerizes with thistrioxane, and a non-volatile protonic acid, to a continuous stirringmixer type reaction device having two parallel shafts which rotate inthe same direction as each other or in opposite directions, a multitudeof paddles mounted on each shaft, and a barrel which comes close to anouter circumference of the paddles, and constituted such that a longaxis end of a paddle periodically comes close to a short axis end of acompanion side, wherein the raw material is charged from an introductionopening provided at one end in an axial direction, and a reactionmixture and an unreacted monomer are obtained from a plurality ofremoval openings provided at the other end; a polymerization reactionstep of carrying out a polymerization reaction of the raw material in astate where the rotating shafts are inclined by 1 to 6° upwards withrespect to a horizontal direction from an input opening of the reactiondevice towards an output opening, and obtaining a reaction mixture; anunreacted monomer re-supply step of vaporizing and separating anunreacted monomer from the reaction mixture, and supplying the same tothe raw material supply step; and a polyacetal copolymer recovery stepof recovering the polyacetal copolymer from the reaction mixture;wherein a weight ratio D/C of the polyacetal copolymer D with respect toa total supplied monomer C which is stipulated as a sum of a monomer Awhich is newly supplied to the reaction device and a monomer B which isrecovered from the reaction device and re-supplied, is no less than 0.7;and a weight ratio D/A of the polyacetal copolymer D with respect to themonomer A which is newly supplied to the reaction device is no less than0.85.

(2) Further, the present invention is a method for producing apolyacetal copolymer as disclosed in (1), wherein a content of theunreacted monomer included in the polyacetal copolymer is no more than1.0 wt %, and the polyacetal copolymer comprises no less than 90 wt % ofparticles with a particle diameter of no more than 11.2 mm.

(3) Further, the present invention is a method for producing apolyacetal copolymer as disclosed in (1) or (2), wherein L/d which isstipulated by a ratio of a length L in a direction of the rotatingshafts of the reaction device with respect to an inner diameter d in aradial direction of the rotating shafts of the reaction device, is noless than 5 and no more than 20.

(4) Further, the present invention is a method for producing apolyacetal copolymer as disclosed in any one of (1) to (3), wherein thenon-volatile protonic acid comprises at least one selected from aheteropoly acid, an isopoly acid, or an acid salt thereof.

(5) Further, the present invention is a method for producing apolyacetal copolymer as disclosed in any one of (1) to (3), wherein thenon-volatile protonic acid is a heteropoly acid shown by formula (1)below or an acid salt thereofH_(x)[M_(m).M′_(n)O_(l) ].yH₂O  Formula (1)

wherein, in the formula (1), M is a central element selected from Pand/or Si, M′ is one or more coordination element selected W, Mo, and V,and 1 is 10 to 100, m is 1 to 10, n is 6 to 40, x is 1 or more, and y is0 to 50.

(6) Further, the present invention is a method for producing apolyacetal copolymer as disclosed in (5), wherein the heteropoly acid oracid salt thereof comprises at least one compound selected fromphosphomolybdic acid, phosphotungstic acid, phosphomolybdotungstic acid,phosphomolybdovanadic acid, phosphomolybdotungstovanadic acid,phosphotungstovanadic acid, silicotungstic acid, silicomolibdic acid,silicomolybdotungstic acid, silicomolybdotungstovanadic acid, or acidsalts thereof.

(7) Further, the present invention is a method for producing apolyacetal copolymer as disclosed in any one of (1) to (3), wherein thenon-volatile protonic acid comprises an isopoly acid shown by formula(2) or (3) below or an acid salt thereofxM^(I) ₂O.pM^(V) ₂O₆ .yH₂O  Formula (2)xM^(I) ₂O.pM^(VI) ₂O₆ .yH₂O  Formula (3)

wherein, in the formulas (2) and (3), M^(I) is hydrogen, but which maybe partially substituted with a metal, M^(V) is at least one selectedfrom V, Nb, and Ta of the V group of the periodic table, M^(VI) is atleast one selected from Cr, Mo, W, and U of the VI group of the periodictable, p and x are 1 or more, and y is 0 to 50.

(8) Further, the present invention is a method for producing apolyacetal copolymer as disclosed in (7), wherein the isopoly acid oracid salt thereof comprises at least one compound selected fromparatungstic acid, metatungstic acid, paramolybdic acid, metamolybdicacid, paravanadic acid, metavanadic acid, and acid salts thereof.

(9) Further, the present invention is a method for producing apolyacetal copolymer as disclosed in any one of (1) to (8), wherein thecomonomer comprises at least one selected from 1,3-dioxolane, diethyleneglycol formal, 1,4-butanediol formal, and ethylene oxide.

Effects of the Invention

According to the present invention, it is possible to economicallyproduce a high quality polyacetal copolymer by a simple process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram for explaining the reaction device 1.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

Below, specific embodiments of the present invention are explained indetail, but the present invention is not at all limited by the followingembodiments, and the present invention may also be practiced with theaddition of suitable modifications which are within the scope of theobjective of the present invention.

The production method of the present invention comprises a raw materialsupply step (S1), a polymerization reaction step (S2), an unreactedmonomer re-supply step (S3), and a polyacetal copolymer recovery step(S4).

Reaction Device 1

For explaining the steps (S1) to (S4), first an explanation will begiven of the schematic constitution of the reaction device 1 withreference to FIG. 1. FIG. 1 is a schematic diagram for explaining thereaction device 1. The reaction device 1 is provided with introductionopening 2 to introduce the raw material, a mixing portion 3 to carry outthe polymerization reaction of these raw materials and obtain a reactionmixture, a vaporization and separation portion 4 for vaporizing andseparating the unreacted monomers from the reaction mixture, and apolyacetal copolymer recovery portion 5 for recovering the polyacetalcopolymer from the reaction mixture.

The mixing portion 3 is a continuous stirring and mixing apparatushaving two parallel shafts which rotate in the same direction as eachother or in opposite directions, a multitude of paddles mounted on eachshaft, and a barrel which comes close to an outer circumference of thepaddles, and constituted such that a long axis end of a paddleperiodically comes close to a short axis end of a companion side,wherein the raw materials are charged from the introduction openingprovided at one end in the axial direction, and the reaction mixture andthe unreacted monomer are obtained from a plurality of removal openingsprovided at the other end. When polymerization-reacting the rawmaterial, the outlet side is elevated, and the rotating shafts areinclined 1 to 6° upwards with respect to the horizontal direction H fromthe input opening (introduction opening 2) of the reaction device 1towards the output opening (polyacetal copolymer recovery portion 5). Ifthe inclination angle (θ) is less than 1°, the retention time of the rawmaterials inside the reaction device 1 becomes too short, and this isnot preferable in the point that there is the possibility that asufficient polymer conversion ratio cannot be obtained. Further, if theinclination angle is less than 1°, the particle diameter of therecovered polyacetal copolymer becomes large, and as a result, theremaining amount of monomer in the copolymer becomes excessive, which isdisadvantageous in economy and quality. If the inclination angle exceeds6°, a considerable burden is applied to the machinery supporting therotating shafts and applying the rotational power, therefore, this isnot preferable because difficulties may arise in the long termoperation.

The mixing portion 3 has a jacket for temperature control, and thetemperature is adjusted by flowing a liquid or gas. It is also possibleto provide a plurality of jackets in the axial direction, andindividually control their temperature.

Further, the size of the reaction device 1 is not particularly limited,but L/D which is stipulated as the ratio of the length L in a directionof the rotating shafts of the reaction device 1 with respect to theinner diameter d in the radial direction of the rotating shafts (thedirection perpendicular to the rotating shafts) of the reaction device 1is preferably no less than 5 and no more than 20. If L/d is less than 5,there is the possibility that a sufficient polymer conversion ratio willnot be obtained, and if it exceeds 20, fluctuations and offsets of theclearance due to deflection of the rotating shafts may readily arise,and this is not preferable from the points of working accuracy andoperational stability.

Further, the clearance between the end of the paddles and the inner faceof the barrel is preferably no more than 2% of the diameter of acircumscribed circle of the paddle, and more preferably no more than 1%.

Further, the rotation speed of the paddles is not particularly limited,but preferably the rotational circumferential speed of the ends of thepaddles is no more than 1.5 m/sec.

Further, the rotation direction of the two rotating shafts may be thesame for both, or may be different directions to each other.

Raw Material Supply Step (S1)

Next, the raw material supply step (S1) will be explained. The rawmaterial supply step is a step of supplying to the above describedreaction device 1 the raw materials comprising trioxane, a comonomerwhich copolymerizes with this trioxane, and a non-volatile protonicacid. Further, for ease of understanding, in FIG. 1, it is disclosedthat a mixture of all of the raw materials is introduced into theintroduction opening 2, but without being limited to this condition, isit sufficient that all of the raw materials are ultimately input to theintroduction opening 2.

[Comonomer]

As the comonomer, a compound selected from cyclic ethers and cyclicformals having at least one carbon-carbon bond is used. Asrepresentative examples of the compound used as the comonomer, forexample, 1,3-dioxolane, diethyleneglycol formal, 1,4-butanediol formal,1,3-dioxane, ethylene oxide, propylene oxide, epichlorohydrin and thelike may be mentioned. Among these, in consideration of the stability ofthe polymerization, 1,3-dioxolane, diethylene glycol formal,1,4-butanediol formal, 1,3-dioxane, and ethylene oxide and the like arepreferable. Further, a cyclic ester, for example β-propiolactone, or avinyl compound, for example styrene and the like can be used. Further,as the comonomer, it is possible to use a monofunctional cyclic ether orcyclic formal having a substituent unit such as butyl glycidyl ether or2-ethylhexyl glycidyl ether. Furthermore, as the comonomer, it ispossible to use a compound having two polymerizable cyclic ether groupsor cyclic formal groups such as a diformal or a diglycidyl ether ofalkylene glycol, for example, butanediol dimethylidine glycerol ether,butanediol diglycidyl ether, and the like, or a compound having three ormore polymerizable cyclic ether groups or cyclic formal groups such asglycerine triglycidyl ether, trimethylol propane triglycidyl ether,pentaerythyritol tetraglycidyl ether, and the like. Polyacetalcopolymers formed therefrom with branched structures or bridgedstructures are also subjects of the present invention.

In the present invention, the amount of the compound selected fromcyclic ethers and cyclic formals for use as the comonomer is 0.1 to 20mol % as a ratio of all of the monomers (the total amount of thetrioxane which is the main monomer and the comonomer), and preferably0.2 to 10 mol %. Less than 0.1 mol % is not preferable because theunstable terminal end portions of the rough polyacetal copolymerproduced by the polymerization increase and the stability worsens, andfurther, a produced copolymer with an excessive amount of comonomerbecomes soft and a reduction in the fusion point occurs.

In the present invention, at the time of polymerizing the above mainmonomer and comonomer and preparing the polyacetal copolymer, in orderto control the polymerization degree, a publicly known chain transferagent, for example a methylal-like low molecular weight linear acetal orthe like may be added.

Further, the polymerization reaction is preferably carried out underconditions where impurities having active hydrogen, for example, water,methanol, formic acid, or the like are not substantially present, forexample, where each is no more than 10 ppm, and for this, it ispreferable to use as the main monomer and comonomer a trioxane, cyclicether and/or cyclic formal which have been prepared so that theseimpurity components are not included, as far as possible.

[Non-Volatile Protonic Acid]

In the present invention, a non-volatile protonic acid functions as thepolymerization catalyst. In the present invention, because anon-volatile protonic acid, and not a boron trifluoride type catalyst,is used as a polymerization catalyst, compared to the case of using aboron trifluoride type catalyst as the polymerization catalyst, thepolymer conversion ratio can be increased.

As examples of a non-volatile protonic acid, compounds comprising atleast one selected from heteropoly acids, isopoly acids, and acid saltsthereof can be mentioned. A heteropoly acid is a polyacid produced by adehydration condensation of different oxyacids, with a specified heteroelement at its center, and has mononuclear or polynuclear complex ionswhich can be formed by condensing condensation acid radicals sharing anoxygen atom. An isopoly acid is a high molecular weight inorganicoxyacid consisting of a condensate of an inorganic oxyacid having apentavalent or hexavalent single type of metal, and is also referred toas an iso polyacid, homonuclear condensation acid, or homogeneouspolyacid.

[Heteropoly Acid or Acid Salt Thereof]

First, the heteropoly acid or acid salt thereof will be explained indetail. A heteropoly acid or acid salt thereof can be shown by theformula (1) below.H_(x)[M_(m).M′_(n)O_(l) ].yH₂O  Formula (1)

A heteropoly acid which is especially effective as a polymerizationcatalyst in the present invention is one where in the abovecompositional formula, the central element M is at least one elementselected from P and/or Si, and the coordination element M′ is one ormore element selected from W, Mo, and V. From the viewpoint ofpolymerization activity, the coordination element M′ is preferably W orMo. Further, in the formula (1), 1 is 10 to 100, m is 1 to 10, n is 6 to40, x is 1 or more, and y is 0 to 50.

Further, an acid salt where the H_(x) in formula (1) is substituted withvarious metals or the like may also be used as the catalyst in thepresent invention.

As specific examples of the heteropoly acid, phosphomolybdic acid,phosphotungstic acid, phosphomolybdotungstic acid, phosphomolybdovanadicacid, phosphomolybdotungstovanadic acid, phosphotungstovanadic acid,silicotungstic acid, silicomolibdic acid, silicomolybdotungstic acid,silicomolybdotungstovanadic acid, and the like may be mentioned. Inparticular, from the viewpoint of polymerization activity, theheteropoly acid is preferably selected from silicomolibdic acid,silicotungstic acid, phosphomolybdic acid, and phosphotungstic acid.

[Isopoly Acid and Acid Salt Thereof]

Next, the isopoly acid or acid salt thereof will be explained in detail.A heteropoly acid or acid salt thereof can be shown by the formula (2)or formula (3) below.xM^(I) ₂O.pM^(V) ₂O₆ .yH₂O  Formula (2)xM^(I) ₂O.pM^(VI) ₂O₆ .yH₂O  Formula (3)

wherein, in the formulas (2) and (3), M^(I) is hydrogen, but which maybe partially substituted with a metal, M^(V) is at least one selectedfrom V, Nb, and Ta of the V group of the periodic table, M^(VI) is atleast one selected from Cr, Mo, W, and U of the VI group of the periodictable, p and x are 1 or more, and y is 0 to 50.

The isopoly acid, besided a method of treating an isopoly acid saltsolution with an ion exchange resin, can be prepared by various methodssuch as adding a mineral acid to a concentrated solution of an isopolyacid salt with ether extraction, and the like. Further, the presentinvention is not limited to an isopoly acid, and an acid salt of anisopoly acid may also be used as a polymerization catalyst. The isopolyacid may be any of the above described formulas (2) and (3), but fromthe viewpoint of polymerization activity, it is preferably an isopolyacid of formula (3) or an acid salt thereof.

As specific examples of suitable isopoly acids, isopoly tungstic acidsexemplified by paratungstic acid, metatungstic acid and the like,isopolymolibdic acids which can be exemplified by paramolibdic acid,metamolibdic acid, and the like, and metapolyvanadic acid,isopolyvanadic acid and the like can be mentioned. Among these, from theviewopoint of polymerization activity, isopolytungstic acid ispreferable.

[Solvent]

In order to carry out the reaction uniformly, the polymerizationcatalyst is diluted in an inactive solvent which has no adverse effectson the polymerization, and is desirably used by adding to the mainmonomer and/or the comonomer. As the above mentioned inactive solvent,an ester obtained from the condensation of a low molecular weightcarboxylic acid with a carbon number of 1 to 10 such as formic acid,acetic acid, propionic acid, butyric acid and the like, and a lowmolecular weight alcohol with a carbon number of 1 to 10 such asmethanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol,2-methyl-1-propanol, 2-methyl-2-propanol, 1-pentanol,3-methyl-1-butanol, 1-hexanol and the like; or a low molecular weightketone with a carbon number of 1 to 10 such as acetone, 2-butanone,2-pentanone, 3-pentanone, 2-hexanone, 3-hexanone, methylisobutyl ketone,methyl-t-butyl ketone and the like, can be preferably mentioned, but itis not limited to these. When also taking into consideration ease ofindustrial availability and the like, methyl formate, ethyl formate,methyl acetate, ethyl acetate, butyl acetate, acetone, 2-butanone,methyl isobutyl ketone and the like are most favorable. Thepolymerization catalyst is dissolved in the above inactive solvent,suitably in a concentration of 1 to 30 mass/wt %, but is not limited tothis. Further, it is also a preferable method to carry out thepolymerization by mixing in advance the above specified amount of thepolymerization catalyst with a portion or all of, one of or a pluralityof the above mentioned main monomer, comonomer, molecular weightmodifying agent and the like, and adding this solution to thepolymerization system.

Polymerization Reaction Step (S2)

Next, the polymerization reaction step (S2) will be explained. Thepolymerization reaction step (S2) is a step of obtaining a reactionmixture by carrying out a polymerization reaction of the raw materialsunder conditions where the rotating shafts are included 1 to 6° upwardswith respect to the horizontal direction H from the input opening(introduction opening 2) of the reaction device 1 to the output opening(polyacetal copolymer recovery portion 5).

It was discovered that in a polymerization reaction step having such aninclination, compared to a BF₃ catalyst of the prior art, a non-volatileprotonic acid such as the above described heteropoly acid can provide ahigher polymerization rate in the reaction device.

The polymerization method is not particularly limited, and for example,the technique disclosed in Japanese Unexamined Patent Application, FirstPublication No. H11-302349 is suitable. In this technique, trioxane, thecomonomer, and the non-volatile protonic acid are held in advance inliquid form and sufficiently mixed, and the obtained reaction rawmaterial mixed liquid is supplied to the polymerization device 1 andcopolymerization is carried out. By this technique, it is possible torestrain the amount of the non-volatile protonic acid and as a result,and this is advantageous in that it is possible to obtain a polyacetalcopolymer with a smaller amount of formal aldehyde emissions.

The polymerization temperature is not particularly limited, but ispreferably carried out at 60 to 115° C., and more preferably carried outat 65 to 110° C. If less than 60° C., there is the possibility that themonomers will separate, and if more than 115° C., there is thepossibility that the monomer will boil and the proportion of polymer inthe finally obtained reaction mixture will be reduced.

Unreacted Monomer Re-Supply Step (S3)

The unreacted monomer re-supply step (S3) is a step of vaporizing andseparating the unreacted monomer from the reaction mixture andre-supplying the same to the reaction device 1.

The technique of vaporizing and separating is not particularly limited,but the activity of the non-volatile protonic acid which is thepolymerization catalyst is extraordinarily high, and it is possible toobtain a high conversion ratio in a short time, thus, from the viewpointof further simplifying these steps, in the vaporization and separationportion 4, a circulation mechanism of reduced pressure, aspiration, orgas flow of inert gas is provided, and vaporization and separation ofthe monomer is carried out.

However, if necessary, any of a method of using a two or more stagepolymerization apparatus where polymerization is carried out up to aprescribed polymerization rate in the first stage polymerization device,and after this, moving to a later stage device, and vaporizing andseparating the unreacted monomer while simultaneously continuing thepolymerization reaction, or a method of deactivating the catalyst underthe presence of a catalyst deactivation agent and concurrentlyvaporizing and removing unreacted monomers, or the like is possible, andthe embodiments may be applied in various other combinations.

In the methods of the prior art, the monomer which was unreacted in thelater stage of the polymerization reaction was removed from the roughpolymer in a later catalyst deactivation and washing step, and wasseparated and collected as a low concentration solution, therefore,before it can be re-supplied to the reaction system, it must beconcentrated and purified, which requires a large amount of energy, andfurther, if the recovery of the monomer is abandoned, it becomes acomplete loss, and either way, it is not possible to avoid a loss in thepolymerization reaction.

On the other hand, in the present invention, the unreacted monomer isremoved without going through steps such as catalyst deactivation orwashing or the like. Therefore, because the unreacted solvent does notinclude a solvent or the like, it can be re-supplied to the reactiondevice 1 while carrying out only an extremely simple purificationprocess. Therefore, there is no polymerization loss of the unreactedmonomer. Namely, the total monomer C supplied to the polymerizationdevice consists of, in addition to the newly supplied monomer A, themonomer B recovered from the polymerization device and re-supplied(C=A+B). Therefore, this has the advantage that, with respect to thenewly supplied monomer, the proportion of the obtained rough polymer isextraordinarily high.

Polyacetal Copolymer Recovery Step (S4)

The polyacetal copolymer recovery step (S4) is a step of recovering thepolyacetal copolymer from the reaction mixture.

The recovered polyacetal copolymer is preferably one comprising 90 wt %or more of particles having a particle diameter of no more than 11.2 mm.This is because, if the particle diameter exceeds 11.2 mm, there is thepossibility that the amount of remaining monomer in the recoveredpolyacetal copolymer will become excessive. Further, in the presentspecification, unless otherwise noted, the particle diameter is taken asthe screen aperture through which the copolymer has passed.

Further, with respect to the recovered polyacetal copolymer, by usingthe technique disclosed in Japanese Unexamined Patent Application, FirstPublication No. 2003-26746, namely, a technique where a solid basiccompound is added to the polyacetal copolymer as-is without carrying outwashing, and undergoing a melt kneading treatment, it is possible tocomplete the deactivation of the polymerization catalyst (thenon-volatile protonic acid) included in the polyacetal copolymer.

By undergoing the above described steps (S1) to (S4), the weight ratioD/C of the polyacetal copolymer D with respect to the total suppliedmonomer C(=A+B) stipulated by the sum of the monomer A newly supplied tothe reaction device 1 and the monomer B recovered from the reactiondevice 1 and re-supplied becomes no less than 0.7. Further, the weightratio D/A of the polyacetal copolymer D with respect to the monomer Anewly supplied to the reaction device 1 become no less than 0.85.

EXAMPLES

Below, the present invention is specifically explained with reference toexamples, but the present invention is not limited thereto.

TABLE 1 Example Comparative Example 1 2 3 4 5 6 7 1 2 3 4 comonomer DOXODOXO DOXO DOXO DOXO DOXO DOXP DOXO DOXO DOXO DOXO polymerizationcatalyst HPA HPA HPA HPA IPA IPA IPA HPA BF₃ BF₃ BF₃ polymerizationcatalyst amount with 2 2 2 2 4 4 4 2 20 20 20 respect to the totalsupplied monomer slope angle (°) 1 2 3 5 2 5 5 0 0 2 5

The abbreviations in Table 1 are as follows. (comonomer)

DOXO: 1,3-dioxolane

DOXP: 1,3-dioxepane

(polymerization catalyst)

HPA: phosphomolybdic acid (non-volatile protonic acid)

IPA: paratungstic acid (non-volatile protonic acid)

BF₃: boron trifluoride (dibutyl ether complex) (well-knownpolymerization catalyst of the prior art)

Examples and Comparative Examples

Using the reaction device 1 shown in FIG. 1, with warm water at atemperature of 85° C. flowing in the jacket, the two rotating shafts aremade to rotate in the same direction at a fixed speed such that therotational circumferential speed of the ends of the paddles is 0.5m/sec, and 3.0 wt % of the comonomer shown in Table 1, and trioxanecomprising 700 ppm of methylal as a molecular weight adjusting agent arecontinuously supplied through the introduction opening 2, and at thesame time, the polymerization catalyst shown in Table 1 is continuouslyadded and adjusted with methyl formate such that the polymerizationcatalyst amount with respect to the total supplied monomers is theamount shown in Table 1, whereby copolymerization of the trioxane andthe comonomer was carried out. When copolymerizing, the rotating shaftswere inclined upwards by only the angle shown in FIG. 1 with respect tothe horizontal direction H from the input opening (introduction opening2) of the reaction device 1 towards the output opening (polyacetalcopolymer recovery portion 5).

Then, the unreacted monomer was vaporized and separated from thereaction system by the vaporization and separation portion 4, led to thecondensation container (not shown in the figure) and collected, andre-supplied to the reaction device 1 by the introduction opening 2 alongwith the newly supplied monomer, and along with this, the polyacetalcopolymer was recovered from the polyacetal copolymer recovery portion5.

Evaluation

In the Examples and Reference Examples, the retention time in thereaction device 1, the polymer conversion ratio of the raw materials,the particle diameter and the remaining amount of the monomer of therecovered polyacetal copolymer were measured.

[Retention Time]

The evaluation of the retention time is measured as the time until 1 wt% carbon black which is supplied by the raw material supply portion 2 isdischarged by the polymer recovery portion 5.

[Polymer Conversion Ratio of the Raw Materials]

The evaluation of the polymer conversion ratio of the raw materials iscarried out by the two types of measurement, of the weight ratio (%) ofthe recovered polyacetal copolymer with respect to the total suppliedmonomer, and the weight ratio (%) of the recovered polyacetal copolymerwith respect to the monomer included in the raw materials. First, theweight A of the monomer included in the raw material, and the weight Cof the total supplied monomer are measured. Next, the obtained productafter the polymerization reaction, after washing with a deactivationagent solution (triethylamine 2 wt % aqueous solution), is dried and theweight D of the thereby obtained polymer is measured. Then, D/C×100, andD/A×100 are calculated. The results are shown in Table 2.

[Particle Diameter of the Recovered Polyacetal Copolymer]

The particle diameter of the recovered polyacetal copolymer wasmeasured. The proportion of particles with a particle diameter of 11.2mm or less, which have passed through a sieve with openings of 11.2 mm,is shown in FIG. 2.

[Remaining Monomer Amount]

The remaining amount of the monomer is determined by washing therecovered polyacetal copolymer with a 2 wt % aqueous solution oftriethylamine, and using gas chromatography to determine the amount ofthe monomer in the washing solution. The weight % of the content of themonomer with respect to the content of the recovered polyacetalcopolymer is shown in FIG. 2.

TABLE 2 Example Comparative Example 1 2 3 4 5 6 7 1 2 3 4 retention time(min.) 8.0 8.1 8.0 8.3 8.1 8.3 8.2 6.7 6.8 8.1 8.3 weight ratio D/C withrespect 0.71 0.72 0.74 0.74 0.72 0.74 0.74 0.62 0.60 0.67 0.67 to thetotal supplied monomer C of the recovered polymer D weight ratio D/Cwith respect 0.85 0.86 0.86 0.88 0.85 0.87 0.87 0.73 not possible notpossible not possible to the newly supplied monomer to recover torecover to recover A of the recovered polymer D unreacted unreactedunreacted monomer monomer monomer 0.601 0.665 0.671 proportion ofparticle diameter 92 92 93 94 93 94 94 80 82 90 91 of 15 → 11.2 mm orless in the recovered polymer remaining monomer content 0.68 0.60 0.580.43 0.65 0.52 0.45 1.2 not possible not possible not possible (wt %) torecover to recover to recover unreacted unreacted unreacted monomermonomer monomer (>10) (>10) (>10)Improvement of the Weight Ratio D/C of the Recovered PolyacetalCopolymer D With Respect to the Total Supplied Monomer C(=A+B)

Upon comparing Examples 1 to 4 with Comparative Example 1, it wasconfirmed that it is possible to lengthen the retention time of the rawmaterials inside the reaction device 1, and as a result of this, theweight ratio D/C can be greatly improved, by carrying out thepolymerization reaction of the raw materials in a state where therotating shafts of the reaction device 1 are inclined 1 to 6° upwardswith respect to the horizontal direction H from the input opening of thereaction device 1 (introduction opening 2) to the output opening(polyacetal copolymer recovery portion 5) to obtain a reaction mixture.Further, it was also confirmed that the particle diameter of therecovered polyacetal copolymer is also small.

Further, upon comparing Example 2 or 5 with Comparative Example 3, orExample 4 or 6 with Comparative Example 4, it was confirmed that theweight ratio D/C can be greatly improved by using a non-volatileprotonic acid as a catalyst for the polymerization reaction.

Recovery of Unreacted Monomer from the Polyacetal Copolymer

In the case of using a non-volatile protonic acid as the catalyst forthe polymerization reaction, it was possible to recover unreactedmonomer from the polyacetal copolymer (Examples 1 to 7). On the otherhand, in the case of using boron trifluoride (dibutyl ether coordinationcompound) as the catalyst of the polymerization reaction, it was notpossible to recover unreacted monomer from the polyacetal copolymer(Comparative Examples 2 to 4). This is because the boron trifluoridetype polymerization catalyst is volatile, and the polymerizationcatalyst which evaporates along with the recovered monomers causes apolymerization reaction, a large amount of polymer is generated, and ablockage of the recovery path for the monomer occurs.

Improvement of the Weight Ratio D/A of the Recovered PolyacetalCopolymer D With Respect to the Monomer A Newly Supplied to the ReactionDevice 1

Upon comparing Example 2 or 5 with Comparative Example 3, or Example 4or 6 with Comparative Example 4, it can be understood that it ispossible to greatly improve the weight ratio D/A by using a non-volatileprotonic acid as the catalyst for the polymerization reaction. In thecase of using boron trifluoride (dibutyl ether coordination compound) asthe catalyst of the polymerization reaction, as described above, it wasnot possible to recover the unreacted monomer (Comparative Examples 2 to4).

According to the production method of the present application, comparedto the method of the prior art, using simple and energetically favorablesteps, it is possible to obtain a suitable polyacetal copolymer in alater step.

EXPLANATION OF REFERENCE NUMERALS

1 reaction device

2 introduction opening

3 mixing portion

4 vaporization and separation portion

5 polyacetal copolymer recovery portion

The invention claimed is:
 1. A method for producing a polyacetal copolymer, comprising: supplying a raw material comprising trioxane, a comonomer which copolymerizes with the trioxane, and a non-volatile protonic acid, to a continuous stirring mixer type reaction device having two parallel shafts which rotate in the same or opposite directions, wherein a multitude of paddles is mounted on each shaft, and a barrel near an outer circumference of the paddles, wherein a long axis end of a paddle periodically comes close to a short axis end of a companion side, wherein the raw material is charged from an introduction opening provided at one end in an axial direction, and wherein a reaction mixture and an unreacted monomer are obtained from a plurality of removal openings provided at the other end; carrying out a polymerization reaction of the raw material in a state in which the rotating shafts are inclined by 1 to 6° upwards with respect to a horizontal direction from an input opening of the reaction device towards an output opening, and obtaining a reaction mixture; vaporizing and separating an unreacted monomer from the reaction mixture, and supplying the same to the raw material supply; and recovering the polyacetal copolymer from the reaction mixture, wherein a weight ratio D/C of the polyacetal copolymer D with respect to a total supplied monomer C which is defined as a sum of a monomer A which is newly supplied to the reaction device and a monomer B which is recovered from the reaction device and re-supplied, is no less than 0.7; and a weight ratio D/A of the polyacetal copolymer D with respect to the monomer A which is newly supplied to the reaction device is no less than 0.85.
 2. The method for producing a polyacetal copolymer according to claim 1, wherein a content of the unreacted monomer included in the polyacetal copolymer is no more than 1.0 wt %, and the polyacetal copolymer comprises no less than 90 wt % of particles with a particle diameter of no more than 11.2 mm.
 3. The method for producing a polyacetal copolymer according to claim 1, wherein L/d which is defined as a ratio of a length L in a direction of the rotating shafts of the reaction device with respect to an inner diameter d in a radial direction of the rotating shafts of the reaction device, is no less than 5 and no more than
 20. 4. The method for producing a polyacetal copolymer according to claim 1, wherein the non-volatile protonic acid comprises at least one selected from the group consisting of a heteropoly acid, an isopoly acid and an acid salt thereof.
 5. The method for producing a polyacetal copolymer according to claim 1, wherein the non-volatile protonic acid is a heteropoly acid shown by formula (1) below or an acid salt thereof H_(x)[M_(m).M′_(n)O_(l) ].yH₂O  Formula (1) wherein, in the formula (1), M is at least one central element selected from the group consisting of P and Si, M′ is one or more coordination elements selected from the group consisting of W, Mo, and V, and 1 is 10 to 100, m is 1 to 10, n is 6 to 40, x is 1 or more, and y is 0 to
 50. 6. The method for producing a polyacetal copolymer according to claim 5, wherein the heteropoly acid or acid salt thereof comprises at least one compound selected from the group consisting of phosphomolybdic acid, phosphotungstic acid, phosphomolybdotungstic acid, phosphomolybdovanadic acid, phosphomolybdotungstovanadic acid, phosphotungstovanadic acid, silicotungstic acid, silicomolibdic acid, silicomolybdotungstic acid, silicomolybdotungstovanadic acid and acid salts thereof.
 7. The method for producing a polyacetal copolymer according claim 1, wherein the non-volatile protonic acid comprises an isopoly acid shown by formula (2) or (3) below or an acid salt thereof xM^(I) ₂O.pM^(V) ₂O₆ .yH₂O  Formula (2) xM^(I) ₂O.pM^(VI) ₂O₆ .yH₂O  Formula (3) wherein, in the formulas (2) and (3), M^(I) is hydrogen which may be partially substituted with a metal, M^(V) is one or more selected from the group consisting of V, Nb, and Ta of group V of the periodic table, M^(VI) is one or more selected from the group consisting of Cr, Mo, W, and U of group VI of the periodic table, p and x are 1 or more, and y is 0 to
 50. 8. The method for producing a polyacetal copolymer according to claim 7, wherein the isopoly acid or acid salt thereof comprises at least one compound selected from the group consisting of paratungstic acid, metatungstic acid, paramolybdic acid, metamolybdic acid, paravanadic acid, metavanadic acid, and acid salts thereof.
 9. The method for producing a polyacetal copolymer according to claim 1, wherein, wherein the comonomer comprises at least one selected from the group consisting of 1,3-dioxolane, diethylene glycol formal, 1,4-butanediol formal, and ethylene oxide. 